Old Faithful, for one (6)
I believe the answer is:
geyser
'old faithful for one' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for geyser that I've seen before include "Spring that discharges steam and hot water" , "wall heater" , "Fountain of hot water from natural spring" , "Natural spout of hot water" , "Spring discharging hot water and steam" .)